Sam Bradford is carted off the field Sunday after suffering a season-ending knee injury. / Bob Donnan, USA TODAY Sports

by Tom Pelissero, USA TODAY Sports

by Tom Pelissero, USA TODAY Sports

EDEN PRARIE, Minn. - A fractured rib opened the door for the Minnesota Vikings to bench Christian Ponder in favor of Matt Cassel. Then they signed Josh Freeman, who was supposed to keep the job until he suffered a concussion in his debut, likely giving Ponder another shot.

An injured thumb opened the door for the Cleveland Browns to bench Brandon Weeden in favor of Brian Hoyer, who was supposed to keep the job until he blew out a knee. Weeden got another shot, then got benched again in favor of Jason Campbell.

By Sunday afternoon, quarterback changes will have been made this season by 11 NFL teams, including the Chicago Bears and St. Louis Rams, who lost their starters last week. Forty-five quarterbacks will have started at least one game â?? and more than half the season remains.

"This is not exclusive to us. There are risks at the quarterback position," Vikings coach Leslie Frazier said Wednesday. "You're going to take some hits. Those guys are in a vulnerable position a lot of times. So, it's a premium on having a good backup because of that."

But why are so many of those backups seeing the field already this season?

In 2012, only three teams made a starting quarterback change in the season's first seven weeks, and there were just six total listings on injury reports for QBs as doubtful or out. In 2011, there were 10 doubtful or out listings at this point. This year, it's 17 and rising fast, with injuries contributing at least in part to every team's initial change except Freeman's benching and release in Tampa Bay.

The evolution toward mobile quarterbacks and poor pass protection may be exposing QBs more than ever. But there's also been a lot of bad luck, from Hoyer's knee sticking in the ground as he slid on a scramble to the Chicago Bears' Jay Cutler tearing his groin on a pedestrian sack.

"You want to make sure you have two or three of those guys," Kansas City Chiefs coach Andy Reid said. "I've had years like that where we've gone through a couple of them. â?¦ Jeff Garcia used to (say) this â?? 'I'm not the second quarterback. It's 1A and 1B.' That's how it works."

RUNNING DEBATE

At least five of the injuries have occurred on runs -- former NFL quaterback Jim Miller noted -- including the torn ACL that ended Rams quarterback Sam Bradford's season on Sunday at the end of a scramble, with a push from Carolina Panthers safety Mike Mitchell.

Buffalo Bills quarterback E.J. Manuel also suffered a knee injury on a hit along the sideline. Oakland Raiders quarterback Terrelle Pryor was sandwiched on a draw. More teams are at least incorporating zone-read concepts into their offenses, which positions the QB as a running threat.

"I don't care if you're an immobile guy like Peyton (Manning) who may or may not run the ball, but let's say you fake a play-action and your quarterback has to scramble â?? it potentially exposes him to one more hit," Miller told USA TODAY Sports.

"We saw Brian Hoyer â?? he was scrambling, he blew out his knee. E.J. Manuel, he hurts his knee. And Sam Bradford â?? just think of him scrambling out of the pocket. I'm not saying all those were pistol-like plays, but if you're a quarterback and you're going to put them in that situation, it could potentially be exposing him to one more hit, and that could be one too many."

Miller also is quick to bring up the disparity in teams' pass protection plans, quarterbacks' knowledge of their checks and audible system and the number of offensive line combinations a player such as Cutler has played behind.

An executive in personnel for an NFL team, speaking on condition of anonymity for competitive reasons, pointed out that only six teams have started the same five offensive linemen in every game this season and young backups coming from college spread offenses aren't ready to step in.

"The splits are so much wider. They're getting rid of the ball that much quicker. Your offensive linemen aren't protecting for as long," the executive told USA TODAY Sports.

"There's not the 2Â½-, 3-second time allotment where the offensive linemen will hold their blocks. I know I'm reaching, but â?¦ I don't know what it is. The offensive line play in the league is just bad."

PASS FAIL

One missed assignment or misplay up front can have devastating consequences, as can holding the ball too long or failing to know where a free rusher is coming from.

Cutler, Ponder, the Tennessee Titans' Jake Locker (hip), the Houston Texans' Matt Schaub (foot/ankle) and the Philadelphia Eagles' Nick Foles (concussion) â?? in the lineup because Michael Vick pulled a hamstring scrambling â?? are among those injured on hits inside the pocket.

"Most (injuries) come off of throws," Reid said. "I think this is the way it's been for a number of years. I don't have the stats. This isn't something new â?? not in my eyes. ... The more you throw the ball, the more chances they have of getting hurt."

In 2010, nine quarterbacks combined for 17 doubtful or out designations through Week 7, and Dallas Cowboys quarterback Tony Romo missed eight games with a shoulder injury beginning in Week 8. In 2008, there were 20 doubtful or out designations, not including the New England Patriots' Tom Brady, who suffered a season-ending knee injury in the opener.

Teams are investing more and more in Plan B at the most important position, though. Six backup QBs signed contracts in March worth at least $3 million a year. That included Cassel, whom Frazier said was signed in part because of Ponder's mobility and thus the risk.

For teams such as the Bears, the loss of an established starter will test a backup plan to keep alive a playoff push. For teams such as the Browns and Vikings, Plan A was shaky enough they moved on with no intention of going back â?? only for another injury to force their hands.

"The college ranks are seeing more guys that are more athletic, that use their feet, and when you use your feet, you're going to get hit a little bit more. They have that contact," Vikings quarterbacks coach Craig Johnson said.

"Some of the guys â?? it's just sheer bad luck. The pass rushers have become really big, strong, fast, and there's a lot of force going on there. Every franchise wants to protect their quarterback. But you've got to weigh the risk and keep playing."